Fiber lasers experience a tremendous growth in industrial applications. In marking, drilling, welding and many other fields it is economically advantageous to use fiber lasers, which have the highest power usage efficiency and lowest running costs among all high power laser systems. In many of these applications, it is useful to limit the heat deposited on the material being processed to avoid thermal damage and burning. Thus, lasers for marking and welding are often operated in a pulsed mode. In contrast to laser diodes that can be pulsed by rapidly altering the driving current, fiber lasers are difficult to gate. Ideally, one wants to accumulate a large amount of energy from the pump sources, and then quickly switch the laser on by altering the quality (Q) factor of the cavity, so that a high power high energy laser pulse is generated, often known as Q-switching.
One way of altering the light guiding properties of an optical fiber is to use internal electrodes within the fiber, and to apply a voltage across these electrodes in order to affect the refractive index properties of the fiber. The influence may be through the electro-optic effect or through induced mechanical stress in the fiber.
As a general background, reference is made to WO 03/005081, which discloses a method of altering the refractive index of an optical fiber by passing an electric current through an internal electrode arranged along the fiber core. Thermal expansion of the electrode induces mechanical pressure on the core, causing the refractive index to change due to the photo-elastic effect. One potential drawback of the technique, however, is that the rate at which the refractive index can be changed is limited, since the principle is based on thermal effects. Moreover, the electrode must at all times be prevented from melting, because the pressure exerted by the electrode on the fiber core more or less disappears if the electrode melts. Therefore, the current passed through the electrode is limited to about 100-200 mA.